Flow of cosmic rays constantly bombards Earth. Primary cosmic rays consist of single protons (about 90% of all cosmic rays) and alpha particles (majority of the remaining 10%). When these primary cosmic rays hit Earth's atmosphere at around 30,000 m above the surface, the impacts cause nuclear reactions, which produce pions. These pions decay into a muon and muon neutrino at about 9000 m altitude. Many muons decay on the way down into neutrinos and an electron while others reach the surface, and there are still enough particles to be detected fairly easily. About 10,000 muons rain down on each square meter of Earth every minute. This flux is approximately uniform over the Earth's surface.
Muons are electrically charged unstable elementary particles with a mean energy of about 4 GeV, which rain down upon the surface of the earth, traveling at about 0.998 c, where c is a speed of light. The muon has an average half-life of 2.2·10−6 s. The angular distribution of the muons is proportional to cos2α, where α is calculated from the vertical direction.
Muon flow can also be generated artificially. U.S. Pat. No. 3,970,936 discloses communications line that employs such muon source. According to U.S. Pat. No. 7,015,475 current size of accelerators for muon generation can be as small as room-size.
Various detection techniques were proposed for muons detectors. Muon detectors described below are presented here for the purpose of proof of the systems feasibility. However it dow not limit the concept of the present invention to this particular type of detectors.
Cloud chambers with supersaturated vapor can be named as the most popular type of detectors. Cloud chambers allow visualization of muon trajectory. If the chamber is equipped with a three-dimensional coordinate system, the muon incident angle and coordinate can be measured.
The most suitable types of muon detectors for the current system are wire chambers and drift chambers (http://universe-review.ca/I15-02-wircounter.jpb). The wire chambers consist of very large number of parallel wires, where each wire acts as an individual detector. A particle leaves a trace of ions and electrons, which drift toward the nearest wire. By marking off the wires which had a pulse of current one can see the particle's path. Several planes of wires with different orientations are used to determine the position of the particle very accurately. A drift chamber has a similar construction shown in FIG. 1. Typically the chamber has two windows 1 and 2, where 1 is a mylar window. Gas pump 3 is connected with the chamber by inlet and outlet pipes 4 and 5. Three wire gratings are inserted between the windows: two cathode wire planes 6 and 7 and a sense wire plane 8 located in between. The wires in the sense plane spaced farther apart than the wires of the cathode gratings. Output 9 yields a signal caused by a muon passing through the chamber. Varying voltages applied from the source 10 to the cathode wires produce a field in which ionization electrons drift at a constant velocity towards the nearest sense wire. The drift time, measured by an electronic “stopwatch” started by a signal from a scintillator 11, is directly related to the distance between the track of the particle and the wire that produces a signal. This greatly increases the accuracy of the path reconstruction.
The coordinate resolution in best muon detectors (such as drift tubes) can be as good as 50 micrometers.
Additionally, a scintillation detector may be used for muon sensing. Such detector has a good spatial resolution. They can be made by forming layers of plastic optical fibers made out of scintillator material coated with a lower refractive index cladding. These can typically have a diameter of 0.5 to 1 mm. The small size of each independent scintillator means that many readout channels (typically tens of thousands) are required, and it is not practical to equip each one with its own photomultiplier. One solution to this is to gather the fibers into a bundle and connect to an image intensifier. This amplifies the light while maintaining an image, which can then be viewed with a CCD camera, and the position on the image associated with a particular fiber.
Since other particles are stimulating the detector as well, a system of two detectors was proposed to avoid false muon detection. Other particles originating from i.e. terrestrial radiation will also cause stimulation, but those particles have too less energy to penetrate both detectors. They will end up either in the first detector or shortly after it. The detection that occurs almost instant in both detectors is considered as a successful detection of a muon. Muons shielding is not limited to above mentioned additional detector; any other types of shielding can be in order to separate muons from other charged particles.
A sandwich of two coordinate detectors located along the muon path allows simultaneous detecting both the incident angle of the muon and it's coordinate. An ensemble of three detectors allows selecting muons and measuring their coordinates and incident angles.
It is known that muons easily penetrate most of the materials. However an increase of the muon deflection is observed when they pass materials with high atomic number Z such as nuclear or gamma-ray-shielding materials. Two materials that can be used to make an atomic bomb: plutonium-239 and highly enriched uranium with at least 20 percent of uranium-235. Since both materials have high Z numbers, both can be detected by muon technique. Probability of muon deflection angle forms a Gaussian function with a zero mean angle and a width that depends on the material Z number. While muon deflection in 10 cm of aluminum is up to about 10 milliradians, it reaches a value of about 80 milliradians in uranium and plutonium (about 10 cm thick specimen).
Current technologies for nuclear material detection are limited to X-ray and Gamma ray equipment. Both systems must be accurately handled, and their emissions properly controlled. There is a need for reliable and safety system to unveil hidden nuclear materials. Muon detection technique provides a safety alternative with improved penetration ability.
Security check point with muon detector may be combined with other sensor equipment. There is a need for simultaneous check of hidden explosives and nuclear materials at the security check point.